In the ceramic capacitor field the capacitors are generally considered to be of three types. The Hi-K capacitors have a high dielectric constant of between about 4,000 and about 15,000, however the dielectric constant is generally not stable with changes in temperature. The second type is the Mid-K capacitor with a dielectric constant of between about 1,400 and about 2,200 and a non-linear change of dielectric constant with temperature change. The third types are the temperature compensating (TC) capacitors, with a dielectric constant between about 10 and about 90, having a small change in dielectric constant with temperature change. Further, the capacitance change is generally linear. There is a need for a TC capacitor that may be fired at low temperatures of below about 1150.degree. C. There also is a need for TC capacitors with a low dielectric constant of between about 12 and about 20.
Multilayer ceramic capacitors are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder, placing thereupon conducting metal electrode layers, usually in the form of a metallic paste, stacking the resulting elements to form the multilayer capacitor, and firing to densify the material and form a solid solution of the constituent dielectric oxides. Barium titanate is one of the dielectric oxides frequently used in the formation of the insulating ceramic layer. Because of the high Curie temperature of barium titanate, however, strontium and zirconium oxides are commonly reacted with the barium titanate to form a solid solution, thereby reducing the Curie temperature of the resulting ceramic material. Certain other oxides, such as manganese dioxide, may also be added to control the dielectric constant of the resulting material by acting as a grain growth control additive.
Because the materials commonly used to produce temperature compensating ceramic capacitors are generally fired to maturity in air at temperatures greater than 1150.degree. C., the metallic electrode layer must be formed from the less reactive, higher melting alloys of the so-called precious metals, such as palladium and silver, palladium and gold, and other similarly expensive alloys well-known in the art. This is necessary in order to prevent either reaction of the electrode with the insulating ceramic layer or melting which might result in discontinuities in the conducting layer. A method of producing a ceramic composition with a low dielectric constant and other suitable properties, which can be fired at temperatures below 1150.degree. C., would permit the use of a less costly electrode material without sacrificing capacitor performance.
It has been proposed in U.S. Pat. No. 4,335,216 to Hodgkins et al that a low firing temperature dielectric composition be formed with a firing temperature of less than about 1150.degree. C. The Hi-K ceramic composition disclosed therein has a low firing temperature and a high dielectric constant that varies with temperature. There remains a need for a temperature compensated type of dielectric that has a low dielectric constant, and low firing temperature.
In U.S. Pat. No. 4,106,075 to Baumann it is disclosed that a temperature compensated dielectric capacitor may be formed based on TiO.sub.2 and/or ZrO.sub.2 and/or compounds of TiO.sub.2, ZrO, Nb.sub.2 O.sub.3, and/or Ta.sub.2 O.sub.5 with oxides of the alkali metals, alkaline earth metals or rare earth metals. The base ceramic is generally doped with a lead zinc borate or lead zinc calcium borate. However, this dielectric has a relatively high dielectric constant. The temperature compensating properties further are not desirable. The values if calculated on FIG. 1 are over 500 ppm.